The present disclosure relates to a sleeve shaped valve cage for receiving a valve member, such as a valve plug, of a control valve of a processing plant, such as a chemical plant, a foodstuff processing plant, a power generating plant, or the like. The present disclosure also relates to a method for operating a control valve of a processing plant comprising a valve cage and a valve member.
Typically, a valve cage is designed to guide the valve member in a displacement direction or motion direction relative to the valve cage between a closed position in which the valve member prevents a stream of process fluid through the control valve, and an open position in which the valve cage together with the valve member releases an opening area for a process fluid stream. The valve cage thereby provides different throttle characteristics or throttle quotas dependent on the displacement position of the valve member. The throttle characteristic can for example be depend on the flow resistance which the opening area of the valve cage released by the valve member exerts a stream of process fluid through the control valve.
When the valve member is arranged in the open position, usually the maximally available opening area or entire opening area of the valve cage is laid open by the valve member, so that the process fluid can flow through the total opening area of the valve cage. Within the valve cage, one or more openings can be provided which are increasingly released during the movement or displacement of the valve member from the closed position to the open position. The flow resistance which the valve member together with the valve cage exert upon the stream of process fluid depends inter alia from the size and shape of those openings which are released in a respective displacement position of the valve member.
Known valve cages traditionally have evenly distributed openings of uniform size. By rendering the valve member displaceably guided within such a classical valve cage, the available opening area of the valve cage can be released essentially proportionally with respect to the displacement of the valve member in the valve cage. Correspondingly the throttle characteristic then changes essentially proportional to the displacement of the valve member in the valve cage, and in case of a constant pressure upstream of the control valve, an increasingly growing mass stream of the fluid stream can flow through the control valve or the provided opening area.
It has been shown that it can be desirable, depending on the area of operation of the control valve to provide the control valve having a constantly high upstream pressure on the one hand with a first control section having a significant flow resistance or significant throttle characteristics, but on the other hand also to provide the control valve with a second control section of low flow resistance. Control valves with a traditional valve cage with evenly distributed openings of constant size, however, in particular at high process fluid pressures, always create a significant flow resistance pressure drop via the control valve.
As a further development of such traditional control valves, improved valve cages have been established which include both a throttle section for a pressure-reduced throughflow of the stream of process fluid which includes multiple throttle conduits extending from the inside of the valve cage to its outside, as well as a high capacity flow section adjacent to the throttle section in the displacement direction which includes equalization channels for large flow rates of the process fluid stream. Such valve cages are described in WO 2011/118863 A1 and WO 2014/070977 A1.
WO 2011/118863 A1 describes a valve cage with a throttle section in which first to third throttle conduits are arranged next to one another, each having an elongate cross-section angled relative to the axis of the valve cage. In the direction of the stream, i.e. radially with respect to the axis of the valve cage, the throttle conduits progress in a step-like or labyrinth-like manner, to thereby provide a pressure-reducing throttle effect. In the axial direction above the throttle section passage windows can be provided which allow a throughflow of large amounts of process fluid through the valve cage and therefore run step-free, rectilinearly in the radial direction through the wall of the valve cage. In the valve cage described in WO 2011/118863 A1, it was shown as disadvantageous that a dead band is present in the section between the axially topmost throttle conduit and the axially adjacent passage window, so that, when displacing the valve plug in the valve cage, in spite of a movement of the valve plug through the dead band, the flow rate of the process fluid stream does not change, which usually leads to over searing of the displacement of the valve plug.
This problem is addressed with the valve cage according to WO 2014/070977 A1. In this valve cage, a transition section is provided between the sound attenuating throttle section in which annular sleeve discs lying on top of each other are provided with radial throttle conduit openings, and an axially adjacent full throughflow section or high capacity flow section, which is formed by a bushing having multiple passage windows. In the transition section, transition openings are realized formed as cuts into the end faces of the passage window sleeve with which the passage window sleeve is set on top of the topmost sleeve disc of the throttle section, so that a dead band is avoided. However, the controllability of a control valve having a valve cage according to WO 2014/070977 A1 still turned out to be unsatisfactory.
The exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings.